Hitherto, liquid developers used for developing electrostatic images have been prepared by dispersing a coloring agent such as carbon black or Nigrosine, etc., a resin for forming toner particles which contributes to control of electric charges or acceleration of dispersion of toner particles by adsorbing in or covering the coloring agent, and, further, to improvement of fixation of images after development, a substance which dissolves in or swells by a liquid carrier to increase dispersion stability of toner particles and a substance which is able to increase the amount of electric charges and stabilizes electric charges on the toner particles, in a liquid carrier having a high electric resistance (10.sup.9 to 10.sup.15 .OMEGA..multidot.cm).
Since electric charges on toner particles have a large influence upon images obtained by development processing, many efforts have been made to stably control them. At the present moment, processes roughly divided into two types have been known.
The first process comprises covering the surface of toner particles with a substance which is ionized or is capable of carrying out adsorption of ions. As substances used for such a purpose, there are oils such as linseed oil or soybean oil, etc., alkyd resins, halogenated polymers, aromatic polycarboxylic acids described in Japanese Patent Publication No. 5944/76, acid group-containing water-soluble dyes described in Japanese Patent Publication No. 12869/81 and aromatic polyamine oxidized condensates described in Japanese Patent Application (OPI) No. 12062/75 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), etc. According to this process, since toner particles have polar groups themselves, it is possible to reduce the amount of ion components in the liquid carrier, and, thus, it is possible to produce developers having excellent development characteristics. However, there are problems, in that fine control of the charge is somewhat difficult, and the amount of charge is greatly reduced by the passage of time, depending upon the particular substances used. Further, particles containing such polar groups in a large amount are generally difficult to disperse, because of having a high cohesive force, and it is necessary to do a special device as shown in Japanese Patent Application (OPI) No. 31739/79.
The second process comprises using a substance capable of dissolving in a liquid carrier to carry out transfer of ions between it and toner particles. Known substances for this purpose include metal soaps such as cobalt naphthenate, nickel naphthenate or cobalt 2-ethylhexanate, etc., metal salts of sulfonic acids such as calcium dodecylbenzenesulfonate, metal salts of petroleum type sulfonic acids or metal salts of sulfosuccinic acid esters, etc., lecithin, polyvinyl pyrrolidone resins, polyamide resins, sulfonic acid containing resins described in Japanese Patent Publication No. 24944/81, and hydroxybenzoic acid derivatives described in Japanese Patent Application (OPI) No. 139753/82, etc.
The second process has been widely used, because addition of the charge controlling substance is easily carried out, and fine control of the charge can be carried out in this way. However, the electric resistance of the developer tends to be reduced thereby, because a substance easily ionizing is generally added. Consequently, the optimum amount added is subjected to very narrow restrictions. If the amount is above the appropriate amount, adverse influences, such as smearing of images or reduction of copy density, etc., tend to occur. Further, in the case of continuously producing numbers of copies, there is a problem in that the so-called fatigue of the developer occurs by accumulation of the charge controlling agent, resulting in deterioration of image density or resolving power. Further, some substances cause deterioration by oxidation, etc., during preservation and lose their charge controlling function.
The present inventors have paid our attention to the second type process. As a result of extensive searching to find ionic substances which dissociate properly in liquid carriers and which are stable with the passage of time, the present invention has been accomplished.